In recent years, electrification of automobiles has been promoted for power saving and reduction in fuel consumption. For example, a system for operating an automatic transmission, a brake, and a steering wheel of an automobile with use of power of an electric motor (motor) has been developed and brought to the market. As an electric actuator for use in such a system, there has been known an electric actuator employing a screw mechanism as a motion conversion mechanism configured to convert a rotary motion of a motor into a linear motion to output the motion (for example, see Patent Literature 1). In this case, a screw shaft forming the screw mechanism forms an output member (output shaft) of the electric actuator.
The electric actuator described in Patent Literature 1 includes a planetary gear speed reducer as a speed reducer, and a rotary motion (output) of a motor is reduced in speed by the planetary gear speed reducer, and then is transmitted to a nut member of a screw mechanism.